For over a century the ancient lost wax method of casting has been used to fabricate dental restorations. The success of this method is due to its simplicity, ease of use, and conservative nature of the process. Utilization of the precious metal used for restorations or prostheses of over 95% is easily obtained by remelting the sprues and button attached to the casting. In order to obtain less than 5% waste, the alloy cannot contain volatile elements or elements that are too readily fluxed into slag so that they are lost while the alloy is molten.
Over the last decade the technology of computer controlled machining or milling, CAD/CAM has increasingly been applied to the fabrication of dental prostheses, in particular those made from ceramics. For the dental laboratory the CAD/CAM process holds the promise of reducing labor expense while maintaining the laboratory's productivity.
The nature of the CAD/CAM operation requires the prosthesis to be milled from a larger body of material. The amount of material in the final product is usually a fraction of the amount of material in the original body. The amount of waste or scrap generated is often on the order of 80 to 90%. The process is cost effective for milling ceramics due to the low intrinsic cost of the ceramic materials. However, such a high proportion of waste is quite intolerable for precious metal alloys. For this reason, the milling process is rarely used for the fabrication of dental prostheses using precious metal. The problem is one primarily of economics.
The nature of the CAD/CAM operation requires that the initial body of material be substantially greater than the finished part. Thus, the user must purchase more alloy than is necessary. While the amount of alloy may be minimized in some cases to reduce cost, the main economic factor is how to handle the waste or scrap from machining.
A few typical dental alloy compositions are:
Au9075650Pt6000Pd2122660Ag110028In.5286Sn0206Ir0.10.100Ru000.10.2ColorYellowWhiteWhiteWhite
These alloys and others like them are all designed to be used for the lost wax method of casting. The alloying elements were chosen so that there is little loss of any particular component during the casting process. Using these alloys in a CAD/CAM milling operation however, would not be economical, since the large amount of scrap cannot be readily remelted into another body for re-use. The scrap material must be refined, that is, the scrap material must first be dissolved in acid and then each element retrieved separately. The refining process itself requires specialized equipment and the recovery of the platinum group elements in particular is quite expensive.
Such factors increase the final cost of the finished milled product, making it prohibitively expensive compared to the lost wax method of casting.
Recently there have been demonstrations of making dental prostheses by laser sintering or “rapid prototyping”. In this technique an article, such as a dental prosthesis, is built up gradually from powder. Powder is placed in a suitable location on a starting substrate and later on the surface of the part being made. A laser or other high energy beam is focused on a spot with sufficient intensity to bond the particles of the powder to form a sold mass. The intensity of the laser spot may be just enough to soften the particles so that they cohere by solid state diffusion and bonding, or the intensity may be enough to melt a small region of the powder for essentially welding the particles together.
Either way, the laser point of focus is scanned over the geometry desired to gradually build up a part of the final geometry. Rapid prototyping has been used to make one-off or unique products by either forming a replaceable mold in which a metal is cast, or by directly forming the part of cohered or welded powder. Although laser has been the preferred approach, it may also be feasible to use an electron beam instead of a laser beam.
Either way of laser sintering or rapid prototyping to form a dental prosthesis directly from metal powder can result in surplus powder on or around the part being fabricated. This scrap powder of precious metal should be recycled to reclaim the precious metals. This invention addresses the economic issue of recycling the scrap from milling or other fabrication technique by providing selected alloying additions to gold so that the gold can be readily recycled.